From article, (With the breakthrough in reluctance machine design these past few years, we may be witnessing a sea change with regards to the powertrain for the electric vehicle market. Given reports about the performance of the Model 3, the reported jump in miles per kWh that owners are reporting over prior Tesla models, along with our 5 easy puzzle pieces, it’s a reasonable bet that Tesla has perfected the reluctance machine and in doing so has pulled an engineering rabbit out of its hat.Tesla Model 3 Motor - Everything I've Been Able To Learn About It (Welcome To The Machine)
Any Tesla enthusiast knows full well that the name originally chosen for what was then Tesla Motors was based on a motor design credited to Nikola Tesla, who lived in the 19th century. Virtually every car that Tesla has produced, from the Roadster to the Model S to the Model X, has been powered by a version of that veritable 3-phase AC induction motor.
Let’s pretend for a moment that you were assigned the job of designing a new motor for the Model 3. Elon Musk has specified that your design must cost less than the Model S motor. You have also been instructed that the motor must not compromise on performance, yet it must be lighter and more efficient than its sibling. What would you do?
You’d think about applying for a job at the McDonald’s down the road from the Fremont factory. No, no — after that. Here’s a hint: You would examine all the different electric motor architectures that exist. In doing so, you would come across a design that’s actually older than Nikola Tesla’s 1892 invention. More than 50 years earlier, the reluctance machine had been invented in 1838. And it’s a surprisingly sweet design. The reluctance machine is simple, it’s efficient, it’s compact in size. And, it’s inexpensive to produce. Yet the reluctance motor sat on the shelf for over a century, suffering from a debilitating disease called Torque Ripple (due to the reluctance machine’s propensity to incur a phenomena know as cogging). Torque ripple simply means that the power output of the reluctance motor fluctuates up and down. Certainly not good for an EV. When you put the pedal to the metal, you want a nice smooth acceleration ramp.
The reluctance machine was partially rescued by the same technology that made it possible to put the induction motor into an electric car — power electronics from Silicon Valley. The reluctance motor is notoriously difficult to control (RPM, determination of rotor position, etc.), but modern inverters and control systems helped overcome that foible. Still, the torque ripple issue remained a challenge even as the 21st century approached. But in poking around, you start to notice some research on the topic taking place in the first decade of this century. You come across a 2011 research paper claiming that the torque ripple issue has been addressed. The researcher had embedded some small rare-earth magnets in the stator of a reluctance motor right along with the existing electromagnets. In doing so, the torque curve had smoothed out. As a bonus, the paper claimed to achieve a 30% boost in power output with the inclusion of the rare-earths. Now there’s some first principles thinking. Whoever first thought of lacing the stator with rare-earths has apparently come up with the greatest marriage since someone thought to sink a chocolate bar into a jar of peanut butter, producing the Reese’s Peanut Butter Cup.
Your thoughts coalesce. With two of the major issues of the reluctance machine having been addressed, you take the plunge and start working with this design. The first thing you are able to do is discard that expensive copper rotor in the legacy motor and replace it with a far cheaper ferrous metal rotor. Probably steel. And probably silicon steel. You just saved a ton of money. Next, although the rare-earths are expensive, they are going into the stator, not on the rotor as with a traditional permanent magnet motor, so you’re going to be augmenting the electromagnets with relatively smallish permanent magnets. Your chosen design has some issues with acoustic noise, but you feel that it’s worthwhile to pursue this design because it’s the simplest and least expensive motor to build, yet highly efficient and powerful (especially with those rare-earths). Good job!
So, the first puzzle piece in the theory that Tesla has put a switched reluctance motor in the Model 3 is the magnets. We know they’re in there, and now we know that one of the latest breakthroughs in motor design is the inclusion of rare-earths in the stator of the reluctance machine. This is huge. It has brought the reluctance machine out of mothballs!
Another clue that the Model 3 motor is not using those rare-earths in a conventional permanent-magnet motor design is that the car does not do regen all the way down to 0 miles per hour. For example, the Bolt has a conventional 3-phase PM motor which allows it to do regen to 0 MPH. I saw this for myself last year when I test drove a Bolt — you can stop without applying the brakes. We’re calling this puzzle piece #2.
Here’s another one: The “dealer” sticker on the Model 3 in the showrooms indicate a “Three phase, six pole, internal permanent magnet motor.” The Tesla induction motor is a 4 pole design, as are many EV motors. Why then a six pole motor? This is a reference to the way the coils on the stator are wired to work with the 3-phase power (three separate power legs). The closer the spacing of the poles, the less time there may be for torque to fall off. It may be Tesla’s way to further smooth torque ripple. That’s puzzle piece #3.
Puzzle piece #4 is that various engineering/motor design publications are starting to talk up the reluctance machine (see article links below). And we are starting to see the reluctance design appear in EVs, such as the Prius. Furthermore, UPS has announced that a switched reluctance machine will be used in a program to convert their fleet to electric power. The company is claiming its implementation of the reluctance motor over other designs will reduce charging times and increase energy efficiency by up to 20% (the company is distancing itself from the use of rare-earth magnets, though). And, in general, industry applications for updated reluctance motor designs are starting to pop. For example, in a recent CleanTechnica article, Software Motor Company (SMC) is declaring that its new reluctant machine design — with what they are calling its own version of “secret sauce” — will save 50% on energy costs over the current induction motors in use at Walmart for HVAC, etc.
Finally, the motor in the Model 3 is indeed smaller than the Model S motor. In a recent Jack Rickard EVTV video examining the Model 3, Jack asserted that the Model 3 motor is actually smaller than even the smaller front motor on the Model S. Yet performance has not been overly compromised. Some owners have reported 0–60 times as quick as 4.8 seconds in their Model 3. That of course is due in part to the 1,000 lb of less weight than the S, but still let’s tentatively call this puzzle piece #5.
Regardless of the exact motor design, Tesla has clearly hit it out of the park with the Model 3’s powertrain. They gave their motor designer team, if not a blank check, a blank whiteboard, and the team came up with a design suitable not only for an affordable electric car, but for the upcoming Tesla Semi as well.)
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